The terms “surface mount” or “surface mounted” technology (SMT) are used in the electronics industry to describe how electronic components and devices are mounted onto a printed wire circuit board and how the signal, power, and control lines, as may be required, are connected to the subject device. In particular, the subject component or device is epoxied or soldered on top of the supporting circuit board, and thus, surface mounted thereon. Conventional surface mount technology typically involves the use of vertical metallic conductive structures to connect the circuitry on the printed wire board up to the functional layer of the surface mounted device. On passive RF and microwave devices, such as a band pass filter, this connection is accomplished at the RF input and at the RF output of the device.
FIGS. 1A-1C illustrate an example, shown schematically, of a conventional band pass filter (BPF) 1 including a ceramic (e.g., dielectric material) substrate 2 that is surface mounted via solder or epoxy, for example, to a surface of a printed wiring board (PWB) or printed circuit board (PCB) according to known methods. The ceramic substrate 2 of the filter 1 has a first surface 2a (i.e., top surface) on which the electronic components of the filter are formed by various methods known in the art, and an opposed second surface 2b (i.e., a bottom surface) that directly interfaces with the circuitry on the PWB/PCB (see e.g., FIG. 2). While the ceramic substrate 2 is shown as a substantially rectangular body having four sides 2c-2f, it should be understood that the shape of the filter substrate 2 is not strictly limited to the illustrated shape, and other suitable shapes known in the art can also be employed.
In the typical BPF 1 shown, the electronic components of the filter include an RF input 3A formed on the first surface 2a proximate the side 2c of the ceramic material layer 2. The RF input 3A is connected to have physical and electrical ohmic (i.e., metal to metal) contact with at least one of a plurality of vertical metallic conductive structures, such as a metallized half-vias (castellations) 6A formed on the surface 2a and extending along the side 2c through the thickness direction of the ceramic filter substrate 2, the central one of which is also connected to have physical and electrical ohmic contact with the corresponding signal connection structure 7A located on the opposed second surface 2b of the filter substrate 2. In that manner, the signal connection structure 7A is ohmically connected with the RF input 3A of the filter 1.
Similarly, the filter 1 also includes an RF output 3B formed on the first surface 2a proximate the side 2d of the ceramic material layer 2. The RF output 3B is connected to have physical and electrical ohmic contact with a vertical metallic conductive structure, such as a metallized half-vias (castellation) 6B formed on the surface 2d and extending along the side 2d through the thickness direction of the ceramic filter substrate 2, a central one of which is also is connected to have physical and electrical ohmic contact the corresponding signal connection structure 7B located on the opposed second surface 2b of the filter substrate 2. In that manner, the signal connection structure 7B is ohmically connected to the RF output 3B of the filter 1.
In between the respective RF input 3A and output 3B, a first impedance matching structure 4A, a plurality of filter sections (as shown, there are four sections 5A-5D), and a second impedance matching structure 4B are also provided, in that order. The impedance matching structures 4A, 4B are known in the art and are device application specific, as one skilled in the art can readily appreciate. These impedance matching structures are needed to ensure proper signal transmission between the RF input 3A and the filter sections 5A-D, and likewise, from the filter sections to the RF output 3B.
It should also be noted that in the prior art structure shown in FIG. 1, substantially the entire bottom surface 2b of the filter substrate 2 is covered by ground plane 9, with the exception of the signal connection structures 7A, 7B and the electrically insulating isolation areas 8A, 8B surrounding the respective signal connections 7A, 7B. It is particularly important that continuous ground layer be provided at least in portions corresponding to the footprint of the filter section(s) and proximate the RF input/output couplings. In reality, the size constraints of the actual devices make it such that the whole second (bottom) surface of the substrate is essentially covered with the exceptions noted above.
There is a demand, however, to increase the pass band frequencies for surface mounted band pass filters in view of particular applications, such as fixed and mobile Wireless Access, Point to Point, mm-Wave communications. This is due to consumer demand for increased data and the subsequent bandwidth requirements of the related microwave equipment.
At higher frequencies, however, such as those above 30 GHz, the presence of vertical conductive structures (such as a metalized via hole, or castellations 6A, 6B as shown in FIG. 1) causes undesirable side effects by launching parasitic (spurious) modes (energy). That is, vertical conductor structures are particularly efficient with respect to coupling energy into waveguide cavity modes (i.e., radiated fields represented by arrows E in the wave guide enclosure 11, as shown in FIG. 2) in the stop band frequency ranges of the filter, where the reflected energy results in high RF current in the vertical conductor structure and high magnetic fields. As shown in FIG. 2, the band pass filter 1 inside the wave guide enclosure 11 is connected to the printed wiring board 10. However, this mechanism also launches energy which was originally in a quasi-TEM mode transmission lines (e.g. micro-strip or grounded co-planar waveguide) and instead couples some of that energy into transverse electric (TE) or transverse magnetic (TM) waveguide modes. This creates a parallel path for the energy to bypass the filter, thus degrading the overall filter performance. These spurious modes are clearly undesirable, and can, in some instances, effectively render the subject surface mount device inoperable.
Accordingly, it would be desirable, therefore, to provide a high frequency band pass filter that is not subject to the drawbacks associated with the prior art structures that include vertical conductive structures within the ceramic substrate at the input and output. In addition, eliminating the need for vertical conductive structures would simplify the manufacturing process and reduce costs by eliminating machining and metallization materials needed to form the vertical conductive structures.